The Submillimeter Array

Circumstellar Disks: Imaging molecules in protoplanetary disks:
the mass and chemical composition of planet-forming gas

   The spatial and spectral distribution of molecules

The SMA allowed some of the first detailed studies of molecular gas in protoplanetary disks. Its high spectral (velocity) as well as spatial resolution allows to precisely locate and study the kinematics of simple molecules like CO (Fig. 3, e.g. Qi et al. 2004, 2006). The kinematics, for example, allowed one of the first constraints on the gas turbulence (Hughes et al. 2011), as well as accurate determination of the masses of the central host stars (Rosenfeld et al. 2012). SMA observations of several spectral lines of a species like CO yield temperature information, which combined with the spatial and velocity information allowed to draw a 3D picture on the morphology of CO gas, and the determination of the CO ‘snow line’, the location at which CO gas freezes onto dust grains in the disk (e.g. Qi et al. 2011, 2013). Finally, SMA detections of optically thin isotopologues of CO (12C16O), such as C18O, allow measurements of CO gas masses. Combined with HD (hydrogen deuteride) measurements by the Herschel space telescope, these for the first time showed that, contrary to what was previously believed, CO may not be a good tracer of the total mass of protoplanetary disks (Favre et al. 2013).

dust mass

Figure 03: Emission from CO gas and its isotopologues 13CO and C18O in the HD163296 protoplanetary disk observed with the SMA (from Qi et al. 2011). Each panel from left (red) to right (purple) shows the emission pattern at different radial velocities, as observed thanks to the Doppler red and blue frequency shift of the spectral lines.

   Chemistry of planet-forming gas

Although CO is the most easily detectable molecule in protoplanetary disks, the SMA has also undertaken searches for other gas species, and detected small molecules and ions such as CN, HCN, HNC, HCO+, N2H+, H2CO, DCN, DCO+, C2H, CS, H2D+ (Oberg et al. 2010, 2011a,b, Graninger et al. 2015, Kastner et al 2015). The observed molecular gas displays a range of morphologies that inform us on the ways molecules are chemically created and destroyed (see e.g. Fig. 4). For example, SMA surveys of several disks such as DISCS (Disk Imaging Survey of Chemistry with the SMA, Oberg et al. 2010) indicated that less luminous stars, which host disks that are colder and more protected from the harsh stellar radiation, are more suitable for active chemistry producing these small molecules (Oberg et al. 2011a). SMA observations of molecules are also informative to address issues like deuteration (Qi et al. 2008) or the degree of ionization in the disk gas (Oberg et al. 2011b).

CK Vul

Figure 04: Emission from molecular gas species from an SMA survey of the TW Hya disk. The grey ellipse shows the resolution of the observations. Courtesy of C. Qi, adapted from Williams & Cieza 2011.